Devices employing liquid crystals have found use in a variety of electrooptical applications, in particular those which require compact, energy-efficient, voltage-controlled light valves, e.g., watch and calculator displays, as well as the flat-panel displays found in portable computers and compact televisions. Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which make them the most promising of the non-emissive electrooptical display candidates currently available. However, slow response can impose limitations for many potential applications. Speed of response becomes especially important in proportion to the number of elements which have to be addressed in a device, and this limits the potential use of some types of liquid crystals.
The modes of liquid crystal displays that are most extensively employed at the present time are twisted nematic (TN), supertwisted birefringence effect (SBE), and dynamic scattering (DS), all employing nematic or chiral nematic (cholesteric) liquid crystals. These devices are based upon the dielectric alignment effects (Freedericksz effect) of the nematic and/or chiral nematic liquid crystal (or mixtures of nematic or chiral nematic liquid crystals) upon application of an electric field. The average molecular long axis of the liquid crystal material takes up a preferred orientation in the applied electric field, the orientation of which is dependent on the sign of the dielectric anisotropy of the material or mixture, and this orientation relaxes upon removal of the applied electric field. This reorientation and relaxation is slow, on the order of a few milliseconds.
Although nematic and chiral nematic liquid crystals are the most extensively employed, there are liquid crystal devices that employ more highly ordered smectic liquid crystals. These devices are also based on the dielectric reorientation of the liquid crystals, and response times are on the order of milliseconds.
A recent advance in the liquid crystal art has been the utilization of tilted chiral smectic liquid crystals, which are also termed ferroelectric liquid crystals, in devices which give microsecond switching. Ferroelectric liquid crystals were discovered by R. B. Meyer et al. (J. Physique 36, 1-69 (1975)), and fluorine-containing ferroelectric liquid crystal materials have recently been developed (see, e.g., U.S. Pat. No. 4,886,619 (Janulis), U.S. Pat. No. 5,082,587 (Janulis), and U.S. Pat. No. 5,262,082 (Janulis et al.)). Ferroelectric liquid crystals possess a macroscopic electric dipole density which is perpendicular to the molecular tilt direction and parallel to the smectic layer planes. This provides a much stronger coupling of the molecular orientation to an applied electic field than is available via the dielectric anisotropy. Furthermore, the coupling is polar, so that reversal of an applied electric field can be effectively used to control molecular orientation.
A high speed optical switching phenomenon was discovered for ferroelectric liquid crystals by N. A. Clark et al. (see Appl. Phys. Lett. 36, 899 (1980) and U.S. Pat. No. 4,367,924). Clark developed a surface-stabilized ferroelectric liquid crystal display (hereinafter, SSFLCD) which enabled bistable operation not possible in any of the device applications described above. The SSFLCD has been recognized as having high potential in regard to information content, viewing angle, contrast ratio, and switching time, but development of the SSFLCD has been hindered by problems with defects in the liquid crystal layer structure. These defects arise due to layer shrinkage upon cooling (through the temperature ranges associated with the tilted smectic mesophases) and the resulting formation of a "chevron" layer structure (see, e.g., the discussion by T. P. Rieker et al. in Phys. Rev. Lett. 59, 2658 (1987) and Ferroelectrics 113, 245 (1991), as well as the discussion by Y. Ouchi et al. in Jpn. J. Appl. Phys. 27, L1993 (1988)). Since the defects result in, e.g., a poor contrast ratio and unstable bistability, researchers have earnestly sought means for avoiding the formation of a chevron structure.
European Pat. Publication No. 405,868 (Fujitsu Limited et al.) discloses a liquid crystal composition which exhibits a bookshelf structure. The composition comprises a mixture of ferroelectric (i.e., tilted chiral smectic) liquid crystal compounds, the mixture containing a predetermined amount of a chiral ferroelectric liquid crystal compound having a specific naphthalene ring structure.
Takanishi et al. describe (in Jpn. J. Appl. Phys. 29, L984 (1990) and Mol. Cryst. Liq. Cryst. 199, 111 (1991)) the spontaneous formation of a quasi-bookshelf layer structure in new ferroelectric liquid crystals derived from a naphthalene ring.
Mochizuki et al. obtain both bookshelf and quasi-bookshelf layer structures by using a "naphthalene base liquid crystal mixture with a rubbed polymer orientation films panel" (see Ferroelectrics 122, 37 (1991)).
Research Disclosure 34573 (1993) discloses ferroelectric liquid crystal mixtures having a high percentage of perfluoroether-containing liquid crystal compounds. The mixtures exhibit a spontaneous bookshelf layer structure and have been formulated with negative layer expansion materials to reduce temperature dependence of the layer thickness.
A bookshelf structure obtained by application of an electric field has been reported by M. Johno et al. in Jpn. J. Appl. Phys. 28, L119 (1989) and is also described in U.S. Pat. No. 5,206,751 (Escher et al.).
A bookshelf structure obtained by oblique vapor deposition techniques has been reported by A. Yasuda et al. in Liquid Crystals 14, 1725 (1993).